Why is my site map on Google search console missing URLs and not indexing pages?

[Master-Vaper]

Why is my site map on Google search console missing URLs and not indexing pages?

I have set up my site map in GSC and it has crawled 4000 URLs…. Great.

however there are still 3000 URLs that have been crawled but not not indexed. When I go to an individual url the page says site map N/a.

why is this happening?

it seems to work for some pages and not others!

what am I doing wrong?

how do I correct this?

thanks for your help.